Unless otherwise indicated herein, the approaches described in this section are not prior art to the claims in this disclosure and are not admitted to be prior art by inclusion in this section.
The 3rd Generation Partnership Project (3GPP) is a globally applicable third generation mobile phone system specification that is a result of collaboration between various groups of telecommunications associations, including the European Telecommunications Standards Institute, the Association of Radio Industries and Businesses/Telecommunication Technology Committee (ARIB/TTC), China Communications Standards Association, and the Alliance for Telecommunications Industry Solutions. 3GPP work is ongoing with Universal Terrestrial Radio Access Network (UTRAN) long term evolution (LTE). The 3GPP RAN2 working group has defined a Discontinuous Reception (DRX) mode to save battery life and resources of user equipment (UE). The main principle in DRX is that the UE behavior is defined relative to the successful decoding of the Physical Downlink Control Channel (PDCCH) by the UE. When the UE is in DRX, the UE is allowed to stop monitoring the PDCCH temporarily. DRX uses one or two pre-defined cycles (long and/or short cycles), at the beginning of which the UE monitors the PDCCH over a certain amount of transmission time intervals (TTIs), according to an “On Duration” Timer. The PDCCH carries downlink (DL) assignments as well as uplink (UL) grants.
Whether the UE is awake (e.g., is monitoring the PDCCH) or asleep after the On Duration period, depends on activity (i.e., possible reception of PDCCH control data during the period). To avoid unnecessary scheduling and to avoid wasting of radio resources, the base station (e.g., eNodeB) should know the state of the UE when transmitting downlink data from the base station to the UE. Thus, a set of clear rules for changing from the active state to DRX and back are defined in Reference [1] (3GPP Technical Specification (TS) 36.321, “Medium Access Control (MAC) Specification,” V10.1.0, March, 2011).
FIG. 1 illustrates a traditional DRX mode which includes a long DRX cycle and a short DRX cycle. The UE turns on its receivers to monitor scheduling information on PDCCH. If no transmission on the PDCCH is detected, i.e., the UE wakes but no transmission on the PDCCH is detected, the UE may turn off its receivers, i.e., go to sleep, to reduce battery consumption. After a sleep period, the UE again turns on its receivers to monitor the PDCCH. The cycle is called a Long DRX cycle. If a transmission on the PDCCH is detected at a time, i.e., the UE wakes and a transmission on the PDCCH is detected, the UE shall continuously monitor the PDCCH, i.e., an Inactivity Timer is started to extend the active time for monitoring the PDCCH. When the Inactivity Timer expires, a short DRX cycle is started, during which the UE wakes with a shorter sleep period than that sleep period used in the long DRX cycle. After the short DRX cycle, the UE enters the long DRX cycle. The DRX mode is important to increase the standby times of mobile devices such as small handsets.
Reference [1] also provides some other parameters, such as drx-Retransmission Timer, mac-Contention Resolution Timer, HARQ RTT Timer and others that used in the DRX mode. FIG. 1 does not show these parameters for simplicity.
With the evolution of LTE system, LTE network switches from a homogeneous network into a heterogeneous network where there are provided a Macro eNB with higher transmission power for coverage purpose and a Pico eNB with lower transmission power for capacity purpose. As verified, the handover failure rate is increased in this heterogeneous network. It is therefore proposed that UE connect to both Macro eNB and Pico eNB concurrently, which is called dual connectivity.
Due to complicacy, some UEs support dual connectivity at L2 and/or L3. In other word, the UE's physical layer can only connect with either Macro eNB or Pico eNB at the same time slot. In order for this type of UE to work in “dual connectivity” scenario, sub-frames have to be partitioned into two sets, where sub-frames within one set are used for communication between the UE and Macro eNB, while sub-frames within the other set are used for communication between UE and Pico eNB. That is, the UE operates in a Time Division Multiplexing (TDM) pattern with Macro eNB and Pico eNB.
One issue as to how to coordinate between the TDM pattern and the DRX behavior for TDM type UE in this dual connectivity scenario occurs, since sub-frame partition is also adopted by the DRX mode as shown in FIG. 1 to save UE's power consumption.